MXPA00003060A - Optical amplifier apparatus - Google Patents

Abstract

There is proposed an optical amplifier apparatus particularly for use in networks distributing signals by optical fibers, comprising first and second parallel optical branches (1, 2), the first optical branch (1) including optical amplifier means (5, 6) for amplifying digital signals and the second optical branch including optical amplifier means (7) for amplifying analogue signals, the optical amplifier means (5, 6) of the first optical path (1) being adapted to amplify bi-directional digital signals and the optical amplifier means (7) of the second optical branch (2) being adapted to amplify unidirectional analogue signals.

Description

OPTICAL AMPLIFIER APPARATUS DESCRIPTIVE MEMORY The present invention relates to a new type of optical amplifier apparatus, especially, but not exclusively, to an optical amplifier apparatus created for use in fiber optic networks. Fiber optics is rapidly penetrating the subscriber access networks for the distribution of CATV (cable television) services. Currently CATV input end stations feed distribution services to a large number of subscribers (> 1000) in networks with abundant divisions in the fiber part, as well as in the coaxial part. The current demand for interactive services, such as telephony and high quality videotelephony, is increasing, but it requires a high bandwidth and a bidirectional link between servers and clients. To implement such interactive services in CATV networks, it is necessary to develop high definition wave division multiplexing (HDWDM) techniques and devices. One of these essential devices is a bidirectional optical amplifier of multiple wavelength that must compensate the losses during the propagation and division together with the link for up and down signals. Currently there is no device available as such.

The present invention provides a new optical amplifier architecture having two parallel optical branches, one for amplifying digital signals and the other for amplifying analog signals. These optical branches are parallel in the sense that they are connected in parallel in such a way that certain signals pass through one of the bifurcations while others pass, possibly simultaneously, through the other. Preferred embodiments of this optical amplifier apparatus allow simultaneous amplification of an analog CATV signal and a plurality of bidirectional multiplexed digital signals, compensating for propagation and division loss along the link in the up and down signals. In particular, it is preferred that the optical amplifier apparatus of the present invention uses erbium-doped fiber amplifying elements (EDFA). Typically, the wavelengths handled by the digital path in the optical amplifier apparatus will be in the shortest wavelength region (1530 to 1545 nm) of the EDFA gain spectrum, while the wavelengths handled by the analogue path of the Optical amplifier will be in the longest wavelength region (1550-1560 nm). The frequency modulation of the analog signal often results in a variation in the frequency of the transmission laser. The use of the band of 1550-1560 nm for the amplification of the analog signal enables the distortion of the analog signal, due to this frequency variation of the transmission laser, to be reduced thanks to the fact that the gain slope of the EDFA element is softer in this part of the spectrum. In one embodiment of the invention, the optical bifurcation that handles the digital signals has a first and second amplification portions (usefully, modalized as respective EDFA coils) and a gain flattening filter configured between these two amplification portions. With this configuration, the position of the gain flattening filter can be chosen in such a way that the best compromise between low noise and optimal output energy for the up and down channels can be achieved. It is also preferred that two lasers are used to drive the amplification means in the parallel analog and digital paths. In this way, the gain of each trajectory can be controlled independently. Particularly it is preferred that each of the lasers drives an amplification means in the digital path, while only one of the lasers drives the amplification means in the analogous path. Preferably, parallel digital and parallel paths are joined at their ends by wavelength division multiplexing / demultiplexing devices. Not only such devices enable digital and analog signals to be separated and recombined as desired, they also reduce the multipath interference induced by telephone crosstalk and improve the noise factors of the analog and digital sections, filtering the amplified spontaneous emission (ASE). It is also desirable to include an optical isolator in the analogous branch, when the latter handles unidirectional signals. To induce the compactness of the apparatus, it may be useful to integrate the first signal divider of the distribution network into the optical amplifier apparatus of the present invention. The additional features and advantages of the present invention will be apparent from the following description of the preferred embodiments thereof, given by way of example, and illustrated with the accompanying drawings, wherein: Figure 1 shows the general construction of a first embodiment of an optical amplifier apparatus in accordance with the present invention. Figure 2 shows the general construction of a second embodiment of an optical amplifier apparatus in accordance with the present invention; and Figure 3 shows the general construction of a third embodiment of an optical amplifier apparatus in accordance with the present invention. As illustrated in Figure 1, the optical amplifier apparatus of the present invention includes first and second parallel optical bifurcations 1 and 2. The first parallel branch 1 is adapted to amplify a plurality of preferably unidirectional or bidirectional digital signals, multiplexed while the second optical path is adapted to amplify a unidirectional analog signal, particularly a relatively broadband signal. The parallel optical branches are joined to their ends by wavelength division multiplexing / demultiplexing (WDM) 3 devices and are driven by the first and second laser diodes 4A and 4B. In the present example, laser diodes 4A and 4B operate at 980 nm and have a power output of 120 mW; however, as will be better understood by one skilled in the art, different pump laser wavelengths will be suitable depending on the active impurifier in the amplifying medium. The WDM 3 devices are bandwidth band dividers of the medium and function to separate and recombine the analog channel and the digital channels. They also reduce the telephone crosstalk induced by multipath interference (MPI) and improve the noise factors of both sections, filtering the amplified spontaneous emission. Such devices are suitable WDMs of fiber, diffraction grating, thin film or other type known in the art. In this embodiment of the invention the parallel bifurcations of the amplifying apparatus use amplifiers of fiber impurified with erbium 5, 6 and 7. The first parallel bifurcation 1 operates in the short wavelength band (blue) (1530 -1545 nm) of the spectrum of erbium gain, while the second parallel bifurcation 2 operates in the long wavelength band (red) (1550-1560 nm) of the erbium gain spectrum. The use of the long wavelength band allows a reduction in the analog signal distortion caused by the frequency variation of the transmission laser (when the latter is internally modulated), as explained above. The first optical branch 1 is preferably bidirectional (that is, there is no optical isolator). This optical branch 1 is used to amplify the ascending and descending digital channels at the same time and, with a channel separation of 0.8 nm (100 GHz) between two adjacent multiplexed ascending and descending channels, up to 8 digital channels (for example 4 ascending, 4 descending) can be accommodated in the 1535-1541 nm window. The wavelength interpolation of the up and down signals makes it possible to reduce the mixing effects of four possible waves between signals that propagate at the same time, although only a small spectrum margin of the gain curve is used. Preferably, the amplification is achieved at the first optical bifurcation using two amplification sections (here coils 5 and 6 EDFA) with a flattening filter 8 located between them. The gain flattening filter works to reduce the gain fluctuation between different digital channels. By using two amplification sections 5 and 6, an optimum position for the gain flattening filter 8 can be achieved, which allows an adequate compromise to be made between the optimal output noise and energy efficiency for the ascending and descending channels. In this example the two amplifier sections 5 and 6 are formed from a first EDFA coil 5.5 m long and a second EDFA coil 12 m long, respectively. It will be apparent that a dual coil arrangement alternatively provides different coil compositions and / or different pumping wavelengths directed towards one or both coils. In the present modality, the pump laser diode 4 A directs the first and second amplification sections 5 and 6 of the first optical path by means of a coupler 13 and the respective wave division multiplexing devices (WDM) 1 1 and 12. The coupler 13 it can be a 3dB coupler or a directional coupler with any desired division coefficient depending on the investment levels sought in coils 5 and 6. The second optical branch 2 is unidirectional and, when this optical amplifier apparatus is used in a network that distributes CATV and digital services, works to amplify the analog CATV signal. This bifurcation includes a single amplification section, modeled in this example as an EDFA coil of 15.5 m in length. An optical isolator 14 is included in this optical bifurcation 2, in such a way that it reduces the effects of subsequent reflection. The pump laser 4B drives the amplification at the second optical branch 2 by means of a WDM coupling device 15. The double pump configuration described above is useful because the gain or output energy of each parallel branch can be controlled in a controlled manner. independent between the bifurcations. In this way in a CATV application, the output power of the CATV signal can change, by changing the pump laser excitation current 4B, without altering the output power and the digital channels. Similarly, if it is desired to change the output power of the digital branch, only the excitation current of the laser pump 4A can be changed. A second embodiment of the optical amplifier apparatus according to the present invention is illustrated in FIG. The second modality has many elements similar to those of the first modality and, in this way, the same reference numbers have been used. As illustrated in Figure 2, the optical amplifier apparatus of the second embodiment integrates the first divider 20 of the distribution network. In this example, the integrated divider 20 is a 4-way divider. Additionally, the pulse configuration of the second mode is different from the first mode. It will be noted that although two laser diodes 24 A, 24 B are still used to drive parallel optical paths, the configuration of these laser diodes has been changed compared to the embodiment of FIG. 1. In particular, in the present embodiment, both copropagation pumping lasers 24 A and 24B are coupled in two amplification sections of the first parallel path 1 by the respective coupling device WDM 1 1 and 12. The pumping laser 24 A is coupled to the first coupling device WDM 1 1 by means of a 3 dB coupler 23 that divides the laser energy provided by the pulse laser 24 A in such a way that substantially half of the output energy from it is fed to the second parallel optical branch 2 by means of an additional coupling device WDM 15 . With the pump configuration of Figure 2, the output power or gain of each of the parallel optical branches can also be independently controlled. If it is desired to change the output energy of the optical branch 2, then the energy of the laser diode 24 is altered and a compensation change is made to the output power of the laser diode 24B. On the other hand, if it is desired to change the output energy of the optical branch 1, then the output power of the laser diode 24B is adjusted alone. The operation of the optical amplifier apparatus illustrated in FIG. 2 is illustrated with reference to the results of certain experiments that have been carried out. All the described experiments were performed at room temperature and at a constant pump energy of 130 mW at 980 nm for the first pump laser 24 A and a pump energy of 80 mW at 980 nm for the second pump laser 24B.

Measurements for the analogous bifurcation The output energy noise factor of the analog amplification bifurcation 2 was investigated by scanning the wavelength of the input signal from 1550 nm to 1560 nm (with a range of 1 nm). For each wavelength, the input energy was successively set to -5 dBm, 0 dBm and + 5dBm, values representative of possible operating points of the amplifier. Under these conditions, the noise factors and the output power were as indicated in Table 1 below.

TABLE 1 (*) for the exit port with the highest level of loss The gain slope of the analog amplification bifurcation was investigated using two signals, one signal was used to saturate the gain in the wavelength of the CATV signal, while the second was used as a weak probe signal to monitor the gain around of the CATV wavelength. The saturation beam was established successively at 1550, 1555 and 1560 nm with a saturation input energy set to -5 dBm, 0-dBm and + 5dBm for each wavelength. For each of these saturation conditions, the wavelength of the probe beam was scanned around the CATV wavelength with an input power of -35 dBm and a local gain slope was evaluated in 1 nm widths. The results are shown in table 2 below.

TABLE 2 It is interesting to observe from Table 2 that the gain slope in each case is less than 0.25 dB / nm, which is typical of type II fibers doped with erbium. Therefore, this architecture is suitable for use in CATV applications, the maximum tolerable value of the gain slope is 0.25 dB / nm in such applications.

Measurement for digital bifurcation With respect to the downward direction, the noise, gain and gain flatness factors were investigated in three steps, using 3 lasers. For all measurements, an upward saturation signal at 1538 nm (average wavelength of digital channels) with an optical input energy of -13 dBm (maximum total upward energy carried by the 4 ascending channels at the input of a 1x4 splitter ) was injected into the amplifier through one of the 1x4 output connectors. First, the downward gain and noise factors were measured with a downward signal at 1538 nm for various saturation input energy values: -7, -10, -20, -30 dBm (see Table 3). Second, a downstream probe signal of -13 dBm at 1535 nm was added at the input port of the amplifier and the 1538 nm signal was adjusted to -8.2 dBm in such a way that the total downward input energy remained constant at -7 dBm. Finally, the signal at 1535 nm was replaced by a signal of -13 dBm at 1541 nm. For these operating conditions, it can be postulated that the average inversion together with the amplifier will lead to a monotonic decrement of the gain from 1535 to about 1541 nm, even in the presence of a gain flattening filter. Subsequently, the gain fluctuation is simply given by the difference in gain between these two wavelengths (see Table 4).

TABLE 3 TABLE 4 With respect to the upward direction, measurements were made the same as for the downward direction. Thus, for all measurements, a downstream saturation signal at 1538 nm with an optical input energy of -7 dBm was injected into the amplifier through the input connector. First, the ascending gain and noise factors were measured with an ascending signal at 1538 nm for various values of saturation input energy: -13, -20, -30 dBm (see Table 5). Second, a downstream probe signal of -19 dBm at 1535 nm was added to an output port of the amplifier and the 1538 nm signal was adjusted to -14.25 dBm, such that the total upward input energy remained constant to -13 dBm. Finally, the signal at 1535 nm was replaced by a signal of -19 dBm at 1541 nm. The gain fluctuation is given as the previous one (see table 6).

TABLE 5 TABLE 6 Gmax (@ 1535 nm) Grnax (@ 1541 nm)? G = G max 'nm NF (*)? = 1535-1541 18. 5 dB 16.1 dB 2.4 dB 6.5 dB < NF < 9.4 dB (*) In Tables 5 and 6, the values of the noise factors NF do not take into account the losses (of the order of 7 dB) associated with the 1x4 splitter at the output of the amplifier. It will be observed from the experimental results described above that the optical amplifier apparatus of the present invention has performance characteristics that are very suitable for an application in a network that distributes multiplexed digital and CATV signals. Although the present invention has been described with reference to two specific embodiments thereof, the invention is not limited to the detailed instrumentation of these two embodiments. On the contrary, numerous modifications and adaptations to the apparatus can be made. For example, the operating wavelengths indicated with more energy and excitation currents of the laser diodes are merely illustrative, other values are also possible. Similarly, the stated lengths of the various EDFA elements are merely illustrative, the appropriate lengths will require adjustments depending on the application and the desired performance. Similarly, pulse configurations different from those illustrated in Figures 1 and 2 are possible. For example, in the embodiment of FIG. 1, instead of using a single pulse laser 4A and a coupler 1x2, 3 dB 13 to drive the two amplification sections 5 and 6 of the digital branch, a pair of diodes can be used. laser and a 2x2 coupler. In this way, providing redundancy to the bifurcation of the digital amplifier is protected against a possible failure (in the case of a failure in one of the pulse lasers, the amplification will still take place, with losses of only 3 dB). Alternatively, Figure 3 shows an embodiment of the invention having a different pumping scheme of Figures 1 and 2. The first pumping laser 4A is coupled to means of the amplifier 5 by means of a WDM coupler 13 'in such a way that the coil 5 is pumped in the direction towards the front (the direction towards the front means from left to right from the view of the reader). The remaining pumping energy of the coil 5 is then directed to bypass the filter 8 via a WDM coupler 1 1 'along the bypass path 22, and is directed towards the coil 6 in the reverse pumping direction (from right to left) via the WDM coupler 12 '. In one aspect of this embodiment, the remaining pump output energy portion directed to the coil 6 is preferably on a 50 to 90% scale of the pumping output energy portion used to pump the coil 5. Most preferably , the remaining pump output energy portion directed to the coil 6 is on a scale of 75 to 85% of the output energy portion of the pump used to pump the coil 5. It is most preferably about 80%. Thus, it is intended that the present invention cover the modifications and variations of this invention as they fall within the scope of the appended claims and their equivalents.

Claims (16)

NOVELTY OF THE INVENTION CLAIMS

1. - An optical amplifier apparatus particularly for use within a network that distribute signals through optical fiber, the apparatus being characterized in that it comprises: first and second parallel optical branches (1, 2), the first optical branch (1) including means of optical amplifier (5, 6) for the amplification of digital signals and the second optical bifurcation including optical amplifier means (7) for amplifying analog signals.

2. The optical apparatus according to claim 1, further characterized in that the means of the optical amplifier (5, 6) of the first optical branch (1) are adapted to amplify the bidirectional digital signals and the optical amplification means (7). ) of the second optical bifurcation (2) are adapted to amplify unidirectional analog signals.

3. The optical amplifier apparatus according to claim 1, further characterized in that the optical amplification means (5, 6) of the first optical branch (1) are adapted to amplify digital wavelength division multiplexed signals.

4. The optical amplifier apparatus according to claim 1, further characterized in that the optical amplification means (5, 6) of the first optical branch (1) comprise the first (5, 1 1) and second (6, 12) and a gain flattening filter (8) disposed between said first and second amplification portions.

5. The optical amplifier apparatus according to claim 1, further characterized in that it comprises first and second pump lasers (4A, 4B) coupled to the first and second optical branches (1, 2) in such a way that the gain or energy The output of each of the parallel optical branches can be controlled independently of that or those of the other optical bifurcation.

6. The optical amplifier apparatus according to claim 5, further characterized in that the first pumping laser (24A) is coupled to the second optical branch (2) and to the first amplification means (5) of the first optical bifurcation ( 1), while the second pumping laser (24B) is coupled to the second amplification means (6) of the first optical branch (1).

7. The optical amplifier apparatus according to claim 1, further characterized in that the amplification means or the portions thereof are constituted by amplifiers of erbium doped fiber (EDFA), the first optical branch (1) uses a region of short wavelength (blue) of the EDFA gain spectrum and the second optical bifurcation (2) uses a long wavelength (red) region of the EDFA gain spectrum.

8. - The optical amplifier apparatus according to claim 7, further characterized in that said short wavelength region of the EDFA spectrum refers to the wavelength range of 1530-1545 nm, and the long wavelength region of the EDFA gain spectrum refers to the wavelength range of 1550-1560 nm.

9. The optical amplifier apparatus according to claim 1, further characterized in that the ends of the first and second parallel optical branches (1, 2) are joined by wavelength division multiplexing (WDM) devices (3) .

10. The optical amplifier apparatus according to claim 1, further characterized in that the second optical bifurcation (2) includes an optical isolator (14). 1.

The optical amplifier apparatus according to claim 1, further characterized in that it further comprises a divider (20) connected to an output of the apparatus.

12. An optical amplifier apparatus according to claim 1, further characterized in that the second optical bifurcation (2) is adapted to amplify a CATV signal.

13. The optical amplifier apparatus according to claim 5, further characterized in that the amplification means (5, 6) both counter-pump by means of an output energy portion of the first pump laser (4A).

14. - The optical amplifier apparatus according to claim 5, further characterized in that the amplification means (5) are pumped forward by an output energy portion of the first pumping laser (4A) and the amplifying means (6) is counter-pumping by a portion of remaining output energy of the first pump laser (4A).

15. The optical amplifier apparatus according to claim 14, further characterized in that the remaining energy output portion of the first pump laser (4A) is approximately 50-90% of the output energy portion of the first laser of Pumping that pumps to the amplification means (5).

16. The optical amplifier apparatus according to claim 14, further characterized in that the remnant energy output portion of the first pump laser (4A) is about 75-85% of the output energy portion of the first laser. pumping that pumps into the amplifying medium (5).